The objective of this project is to determine the exact three-dimensional crystal and molecular structure of fructose-1,6-diphosphatase (FDPase) by X-ray diffraction analysis of high resolution (2.5-2.8 A ). Native (neutral) rabbit liver FDPase, which has a molecular weight of approximately 144,000 daltons and is comprised of four identical subunits, is the form of the enzyme being studied. It has been found to crystallize in space group 1222 with one subunit comprising the asymmetric unit (molecular weight equals 36,000). The conventional techniques of isomorphous replacement will be used to solve the structure. The crystallization procedures being used are the microtechniques of hanging drop equilibration and vapor diffusion in multiple depression plates. These techniques allow for the growth of large single crystals from the minimum of material. Precession photography is being used to evaluate the quality of isomorphous derivatives, but intensity data are being collected on a diffractometer. Diffractometer data are more accurate, and the small unit cell size of rabbit liver FDPase makes its use practical. FDPase is one of the key regulatory enzymes in carbohydrate metabolism. This regulation occurs through interaction with a variety of allosteric effectors including AMP, activating and inhibitory divalent cations, and monovalent cations. The studies proposed here will pave the way for eventual X-ray analysis of enzyme-substrate, enzyme-cofactor, and enzyme-inhibitor complexes leading to a precise mapping of the elucidation of the mechanism of action of this important regulatory enzyme, and perhaps provide models for the control of other metabolic processes.